Efficient Solution‐Processed Bulk Heterojunction Solar Cells by Antiparallel Supramolecular Arrangement of Dipolar Donor–Acceptor Dyes

Abstract

Research on small-molecule-based organic semiconductors has undoubtedly been strongly influenced by xerographic photoconductors like triarylamines, the first important organic electronic materials in market products.[1] Their development was strongly influenced by the Bässler model, which provided a rationale for the design of amorphous organic photo-and semiconductors.[2] According to this model, only compounds that lack dipole moments are considered promising for charge-carrier transport because the increased energetic disorder associated with dipole moments is thought to impede charge hopping. Recently, we questioned this paradigm in the field of organic photovoltaics (OPV) and successfully implemented highly dipolar merocyanine dyes as active components for light harvesting as well as exciton and hole transport in solution-cast bulk heterojunction (BHJ) solar cells.[3] The rationale behind our concept [4] was that highly dipolar donor-acceptor (D-A) substituted p systems (also called push-pull dyes) self-assemble into centrosymmetric dimers, [5] thus effectively eliminating molecular dipole moments on the supramolecular and material levels.[6] Two drawbacks of our BHJ materials, however, limited the acceptance of our concept so far. Firstly, the best solar cells were obtained for merocyanine dyes whose molecular scaffolds were equipped with rather bulky substituents that interfere with close face-to-face antiparallel dimerization.[3] Secondly, the power-conversion efficiencies (h) under standard AM1.5, 100 mW cm À2 simulated solar illumination conditions for solution-cast BHJ cells with fullerenes-although significantly advanced by more sophisticated vacuum processing [7] -could not be improved beyond 2.6 %, which is significantly lower than the best solutionprocessed small-molecule-based BHJ devices fabricated with A-D-A and D-A-D chromophores, for example, acceptorsubstituted oligothiophenes (up to 3.7 %) [8] and triarylamines (up to 4.3 %), [9] diketopyrrolopyrroles (up to 4.4 %), [10] and squaraines (up to 5.2 %).[11] Herein, we introduce dipolar D-A dyes with flat structures that undoubtedly form centrosymmetric dimers [5] with perfectly cancelled dipole moments in the solid state. Solution-processed BHJ solar cells derived thereof exhibit power-conversion efficiencies up to 4.5-5.1 % (dependent on light intensity), clearly placing D-A dyes now among the top-performing small molecules in the field of organic photovoltaics.Scheme 1 outlines the synthetic route that follows our earlier work on merocyanine dyes for photorefractive materials [12] and the simple access to 5-dialkylamino-thiophene-2-carbaldehydes by Hartmann.[13] Detailed synthetic procedures and characterization data are described in the Supporting Information.The optical properties of the synthesized dyes were investigated by UV/Vis and electro-optical absorption spectroscopy.[14] Furthermore, cyclic voltammetry was performed for each dye to obtain information about their highest occupied molecular orbital (HOMO) and lowest unoccupied molecu...

Light‐Harvesting Materials for Organic Electronics

“…Following the initial excitation of the ZnP, energy transfer occurs from the singlet excited state of the zinc porphyrin to the free base porphyrin. Then, electron transfer from H 2 P to the fullerene followed by a sequential electron transfer process render Fc •+ -ZnP-H 2 P-C 60 •− as the final charge-separated state, showing an outstanding lifetime of 0.38 s in benzonitrile, which is similar to that observed in natural photosynthesis from bacterial photosynthetic reaction centers.…”

“…9.2). Light irradiation promotes one electron transfer from the zinc porphyrin (ZnP) to the [60]fullerene electron acceptor. The lifetime of the formed charge-separated radical ion pairs is in the range of 415-850 ns in THF and benzonitrile, which may be explained by the changes on the substitution pattern of the triazole linker.…”

“…Porphyrins have also been used to prepare larger photoactive materials. Thus, Imahori et al [12] have described the preparation of tetrad 3, where the correct position of the different photoactive units generates a redox gradient from the ferrocene unit (Fc) to the [60]fullerene ( Fig. 9.3).…”

Effects of oligothiophene π-bridge length on physical and photovoltaic properties of star-shaped molecules for bulk heterojunction solar cells

“…Appearing steric limitations lead to the azimuthal irregularities in stacking of the molecules along the column, as every molecule should adjust its position not only with its top and bottom neighbors inside the cylinder, but with its side neighbors as well. An interesting feature of the columnar mesophase in compound N(Ph-3T-DCN-Hex) 3 are the specic relative intensities of the reections: second reection (11) is 7.5 times stronger than the rst one (10), and the third one (20) is of the same intensity (I 10 ¼ I 20 ). Thus, with temperature increasing to 140 C (Fig.…”

Effects of Heteroatom Substitutions on the Crystal Structure, Film Formation, and Optoelectronic Properties of Diketopyrrolopyrrole‐Based Materials

“…To effi ciently absorb solar irradiation, the conjugated backbone usually incorporates chromophoric moieties such as benzoporphyrin, [ 6 ] diketeropyrrolopyrrole (DPP), [7][8][9][10][11] isoindigo, [ 12 , 13 ] squaraine, [14][15][16][17][18] oligothiophene, [19][20][21][22][23] borondipyrromethene, [ 24 , 25 ] and merocyanine. [26][27][28] The conjugation of the backbone is typically extended over several aryl moieties and ended by groups such as vinyl, thienyl, and phenyl derivatives. [ 3 , 4 ] Given the vast combinations of different conjugated moieties, detailed knowledge of how specifi c chemical moieties and structures translate to material properties remains limited.…”